In a process for forming large, substantially flat articles in an industry such as the automobile industry it is common to use large conventional presses to stamp or press body panels such as roofs, hoods and trunk lids from sheet metal.
Such a stamping or pressing process usually involves a press with a three piece tool which has two large shaping surfaces, typically two halves of a mould made of highly expensive tool steel. There is also a third device to hold a sheet of metal, which is subsequently shaped by the two mould halves.
Depending on the degree of deformation involved, there may be more than one stamping or pressing stage needed to achieve the required shape and profile. However a conventional press is limited to achieving a maximum of between 1%-2% stretching during the shaping of substantially flat articles.
Membrane presses have been used extensively for the forming of articles from sheet and U.S. Pat. No. 2,344,743 and U.S. Pat. No. 3,614,883 describe membrane presses in which a blank sheet of metal is formed against a single forming surface. It is further described that the edges of a blank may be trimmed during a press cycle by being forced into a recess. However, the degree of deformation available with the devices of U.S. Pat. No. 2,344,743, U.S. Pat. No. 3,614,883 is limited to what may be achieved in localized parts of the forming surface by the fixed contours of the forming surface. U.S. Pat. No. 3,566,650 describes a membrane press in which a tool or plunger may be incorporated in a forming tool to shape or deform a blank in one place during a press cycle. U.S. Pat. No. 3,672,194 describes a membrane press in which the edges of a blank may be trimmed or perforated during a single pressure cycle.
EP 0 288 705 A2 describes a membrane press in which a metal blank may be formed against a one sided forming tool. Additionally the tool comprises moving parts which are moveable during a press cycle that includes increasing pressure through various phases. During a cycle edge cutting may be followed by drawing material into recesses of the mould tool, and intermediate cutting may be then carried out by further deep drawing into undercuts, before a finishing pressure is attained. EP 0 288 705 A2 describes how local shaping may be carried out in a continuous operation.
An article entitled "A Giant among Presses" in a Swedish publication Verkstaderna nr 8 dated Aug. 12, 1991 describes a Swedish membrane press of the Quintus type and a forming tool used by an automobile manufacturer. The use of a membrane press for short run production of articles is discussed in the article and it is stated that an integration of trimming edges in a forming tool is especially advantageous in short run production.
However neither EP 0 288 705 A2 nor any of the above references describe how extensive and uniform plastic deformation of a blank may be achieved, extensive deformation greater for example than 1%-2% stretching during the shaping of substantially flat articles, nor a device for achieving such a deformation, and neither is it described how such extensive plastic deformation may be combined with other processes in a single press operation.
In conventional processes, additional shaping stages may be required to cause sufficient plastic deformation to result in work hardening in the sheet metal and thereby achieve a particular strength in the metal. Additional stages are usually required for trimming surplus material. After pressing and trimming the edges the shaped article is ready for surface treatment as part of a painting process.
The conventional stamping or pressing process needs further improvement. In particular to produce high strength, low weight, substantially flat articles from sheet metal without additional shaping stages. It would also be preferable for a stamping or pressing process to be able to handle ready treated or even ready decorated sheet metal. In addition, it would be beneficial if the cost of the tooling for such a shaping process could be reduced so that substantially flat metal articles may be produced in low volume production runs to supply niche or specialised markets.